Actuator

ABSTRACT

An actuator, preferably a linear actuator for furniture, comprises a brake spring ( 20 ) in the form of a helical spring having a plurality of windings wound around a cylindrical element ( 10 ) of plastics rotatable at least during braking, said spring being tightened around the cylindrical element for braking. The frictional heat generated hereby can cause the plastics to be deformed, which adversely affects the braking power. This is solved by providing the cylindrical element ( 10 ) consisting of plastics with an insert ( 12 ) of metal for carrying off the frictional heat preferably into other metal parts of the actuator that may serve as cooling faces. To carry off heat additionally, it is proposed to arrange a second heat-conducting element ( 18 ) in intimate contact with the outer side of the spring ( 20 ), said element being likewise connected with other metal parts of the actuator that may serve as cooling faces.

BACKGROUND OF THE INVENTION

The present invention relates to an actuator, preferably for furniture,and comprising a holding brake spring in the form of a helical springhaving a plurality of windings around a cylindrical element of plasticsrotatable at least during braking, the spring being tightened around thecylindrical element for braking.

THE PRIOR ART

EP 662 573 B1 discloses a linear actuator of the type mentioned above.To illustrate the background of the invention, reference is made to theembodiment shown in FIG. 2 of the EP document. The shown actuator isdriven by an electric motor which drives a spindle via a worm drive. Thespindle has positioned thereon a nut to which an extension rod issecured. The worm is formed in extension of the motor shaft and is ofsteel. On the side of the worm wheel there is provided, as an integratedpart of it, a cylindrical element around which a brake spring ispositioned whose one end is fixed to the motor housing. The spindle andthe worm wheel are interconnected by a splined connection, of which onepart is provided in a cavity in the cylindrical element, while the otherpart is an element on the end of the spindle. For a low level of noiseand small wear on the worm as well as for reasons of costs, the wormwheel is moulded in plastics.

During braking, the kinetic energy of the spindle is converted intofrictional heat in the spring and the cylindrical element. Thegeneration of heat can be rather violent with a consequent risk ofdeformation of the plastics material, which adversely affects thebraking effect. It might then be contemplated to make worm wheels ofmetal, but this involves unacceptable transmission noise, just as thedegree and the uniformity of the braking are impeded. Anotherpossibility is then to use a plastics material having betterheat-resistant properties, such as fibreglass-reinforced plastics, butthese involve an unacceptably great wear on the worm. Air and liquidcooling are excluded in advance, partly because of costs, and partlybecause of insufficient space in the actuator.

SUMMARY OF THE INVENTION

According to this invention the problem is solved by providing theplastic cylindrical element with an insert of metal for carrying off thefrictional heat generated during the braking. Hereby it is possiblestill to use the plastics material with the advantages involved by this,while the heat problems of the plastics can be avoided in a relativelysimple way. The metal insert may be positioned right under the spring,i.e., precisely where the heat is generated, and the plastics layerbetween insert and spring may be made very thin so that the heat isconducted directly to the insert. The plastics may be moulded directlyon the metal insert, and various steps may be taken, such as knurlingof, recesses in, bosses on the surface of the insert, etc., to ensurethe transfer of the forces occurring.

The effect of the carrying-off of the heat may be increased by causingthe insert to contact cooling faces of metal, preferably other actuatorparts consisting of metal, such as a metal frame, a motor housing, aspindle, etc.

In an embodiment of a linear actuator where the connection between wormwheel and spindle is formed by a spline, the insert is formed with thespline so that there is direct contact between insert and spindle. Theheat is thus carried off through the spindle via the insert.

In a further development of the invention the heat is carried offthrough a further element (collar) in intimate contact with the outerside of the spring, the element being made of a more heat-conductingmaterial than the spring. It will be appreciated that the element mustnot interfere with the function of the spring, of course.

In an embodiment for optimum carrying-off of heat, the element coversthe entire or substantially the entire outer side of the spring.

Here, too, the cooling effect may be increased by causing the element tocontact cooling faces, preferably other actuator parts consisting ofmetal.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be explained more fully below withreference to the accompanying drawing. In the drawings:

FIG. 1 shows a longitudinal section through a linear actuator accordingto the invention,

FIG. 2 shows an exploded view of the actuator in FIG. 1, and

FIG. 3 shows a perspective view of the worm wheel on an enlarged scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The linear actuator shown in the drawing comprises a reversible low voltelectric motor (DC) 1, which drives a spindle 2 via a worm drive. Thenumeral 3 designates a front member for the motor. A nut 4 is providedon the spindle and has secured thereto a drive rod 5 in the form of apipe telescopically arranged in an outer pipe 6, which is embedded inthe housing 7 of the actuator.

The worm drive comprises a worm 8 formed in extension of the motorshaft, which is of steel, as well as a worm wheel 9 with a cylindricalelement 10 which is integrated on the side, and which accommodates aninsert 12. The worm wheel is of plastics (POM), fixedly moulded on theinsert which is of lightweight metal (aluzinc).

The worm wheel 9 is secured to the spindle by a splined connection,where one part is formed in the insert 12, and the other part is formedby an element 13 secured on the end of the spindle. The spindle ismounted in a bearing 14 in front of the worm wheel.

On the cylindrical element there is a brake spring in the form of ahelical spring 20 having one end fixed. For this purpose, the end 15 ofthe spring is bent outwards and is received in a hole 16 in an element17, which is secured by a screw to the motor housing. A heat conductingelement collar 18 of copper is provided externally on the spring inintimate contact with it, and is secured with the end via an eye 19 tothe motor housing via the element 17, there being provided a pin herefor hooking-on the element.

The windings of the spring extend around the cylindrical element 10 suchthat the spring does not counteract the spindle in the direction ofrotation when the drive rod 5 is extended, but when the direction ofrotation is reversed for reversal of the drive rod, the spring affectsthe cylindrical element with a braking force. When the drive rod isextended, the direction of rotation of the spindle causes the spring,which is tightly engaged with the cylindrical element, to openfiguratively speaking, as the friction from the cylindrical element actsagainst the winding direction of the spring, and thereby the spring doesnot, or only to an insignificant extent, affect the rotation of thespindle. During reversal, on the other hand, the spring will betightened around the cylindrical element and exert a braking or ratherholding force on the cylindrical element and thereby on the spindle. Anon-self-locking spindle may thus be made self-locking, just as theself-locking ability of a self-locking spindle may be increased, cf. EP662 572 B1.

Part of the frictional heat, which is generated by holding the spindle,is transferred into the insert and via the splined connection into thespindle, just as part of the heat is transferred into the shield andfrom there into the motor housing. The insert and the spindle as well asthe heat-conducting element and the motor housing thereby serve ascooling faces for the holding brake and thereby prevent the plasticsfrom being superheated, thereby avoiding heat deformations of theplastics.

With the measures stated here, it has been shown that it is possible tolower the temperature in the brake from about 160° C. to about 80° C.,which is sufficient to prevent the plastics material (POM) from beingheat-deformed.

The electrical equipment with end stop, control electronics, operatingpanel and power supply is not described, because this is generallyknown.

Described here is a linear actuator which is subjected to a compressiveload. In case of linear actuators which are affected by tensile loads,the winding direction of the brake spring must be reversed of course.

It will be appreciated that the invention may also be applied where thebrake is not mounted directly in the transmission from the motor to theactuation element, but is placed laterally thereof.

1. An actuator comprising a spindle, a worm wheel connected to thespindle by a spline, a plastic cylindrical element which is rotatable atleast during reversed movement, a helical spring having a plurality ofwindings around the plastic cylindrical element for tightening aroundthe cylindrical element during reversed movement, and a metal insertinside the cylindrical element for carrying off frictional heatgenerated during the reversed movement, the metal insert being connectedto cooling faces of metal and the spline being formed in the insert sothat there is direct contact between the insert and the spindle.
 2. Anactuator according to claim 1, including a collar in intimate contactwith an outer side of the spring for carrying off heat, said collarbeing made of a more heat-conducting material than the spring.
 3. Anactuator according to claim 2, wherein the collar essentially covers theentire outer side of the spring.
 4. An actuator according to claim 3,wherein the collar is connected with metallic cooling faces.
 5. Anactuator comprising a helical spring having a plurality of windingsaround a plastic cylindrical element which is rotatable at least duringreversed movement, said helical spring being tightened around thecylindrical element during reversed movement, a metal insert inside thecylindrical element for carrying off frictional heat generated duringthe reversed movement, and a collar in intimate contact with an outerside of the spring for carrying off heat, said collar being made of amore heat-conducting material than the spring.
 6. An actuator accordingto claim 5, wherein the collar essentially covers the entire outer sideof the spring.